The demand for rechargeable batteries having ever greater energy density has resulted in substantial research and development activity in rechargeable lithium batteries. The use of lithium is associated with high energy density, high battery voltage, long shelf life, but also with safety problems (eg. fires). As a result of these safety problems, many rechargeable lithium battery electrochemistries and/or sizes are unsuitable for use by the public. In general, batteries with electrochemistries employing pure lithium metal or lithium alloy anodes are only available to the public in very small sizes (eg. coin cell size) or are primary types (eg. non-rechargeable). However, larger rechargeable batteries having such electrochemistries can serve for military or certain remote power applications where safety concerns are of somewhat lesser importance.
Recently, a type of rechargeable lithium battery known as lithium-ion or `rocking chair` has become available commercially and represents a preferred rechargeable power source for many consumer electronics applications. These batteries have the greatest energy density (Wh/L) of presently available conventional rechargeable systems (ie. NiCd, NiMH, or lead acid batteries). Additionally, the operating voltage of lithium ion batteries is often sufficiently high such that a single cell can suffice for many electronics applications.
Lithium ion batteries use two different lithium insertion compounds for the active cathode and anode materials. The excellent reversibility of this lithium insertion makes such compounds function extremely well in rechargeable battery applications wherein thousands of battery cycles can be obtained. 3.6 V (average) lithium ion batteries based on varied LiCoO.sub.2 /carbon electrochemistries are now commercially available. A wide range of carbonaceous compounds is suitable for use as the anode material, including coke and pure graphite. Also, many other lithium transition metal oxide compounds are suitable for use as the cathode material, including LiNiO.sub.2 and LiMn.sub.2 O.sub.4. The aforementioned products employ non-aqueous electrolytes comprising LiBF.sub.4 or LIPF.sub.6 salts and solvent mixtures of ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, and the like. Again, numerous options for the choice of salts and/or solvents in such batteries are known to exist in the art.
Lithium ion batteries can be sensitive to certain types of abuse, particularly overcharge abuse wherein the normal operating voltage is exceeded during recharge. During overcharge, excessive lithium is extracted from the cathode and a corresponding excessive insertion or even plating of lithium occurs at the anode. This can make both electrodes less stable thermally. The anode becomes less stable as it gets doped or plated with reactive lithium while the cathode becomes more prone to decomposing and evolving oxygen (see J. R. Dahn et al., Solid State Ionics, 69(3-4), p265-270, 1994). Overcharging also results in heating of the battery since much of the input energy is dissipated rather than stored. The decrease in thermal stability combined with battery heating can lead to thermal runaway and fire on overcharge.
Battery chargers and/or battery packs comprising assemblies of individual lithium ion batteries are generally equipped with appropriate electrical circuitry to prevent overcharge from happening. However, in the event of a failure or a deliberate defeating of the circuitry, many manufacturers have decided to incorporate additional safety devices, in the individual batteries themselves, as a greater level of protection against overcharge abuse. For instance, as described in U.S. Pat. No. 4,943,497 and Canadian Patent Application Serial No.2,099,657 respectively, some of the present products of Sony and Moli Energy (1990) incorporate internal disconnect devices which activate when the internal pressure of the battery exceeds a predetermined value during overcharge abuse. Various gassing agents (eg. cathode compounds and/or other battery additives) may be used to generate sufficient gas above a given voltage during overcharge so as to activate the disconnect device. Another approach has been to incorporate overcharge safety devices in the batteries themselves which limit the charging current and/or voltage. For instance, positive temperature coefficient resistors (PTCs) are incorporated by some manufacturers in part to limit the charging current during overcharge abuse. These devices rely on the combination of heating of the battery and IR heating of the PTC to activate the PTC, thereby increasing its resistance and limiting the charging current.
Other means for limiting charging current in the batteries themselves is disclosed in co-pending Canadian Patent Application Serial No. 2,156,800, filed Aug. 23, '1995, by a common applicant. Therein, a small amount of a suitable polymerizable monomer additive is mixed in the liquid electrolyte for purposes of protecting a rechargeable lithium battery during overcharge. The additive polymerizes at voltages greater than the maximum operating voltage of the battery (ie. during overcharge abuse), thereby forming a blocking polymeric film and resulting in an increase in the internal resistance of the battery. As with PTC devices, this increase can limit the charging current sufficiently for protection. Several suitable monomer additives were identified for use in common lithium ion battery electrochemistries. A preferred additive was biphenyl which was shown to provide satisfactory overcharge protection without adversely affecting cycle life of certain battery embodiments tested at 21.degree. C. up to normal maximum operating voltages of 4.2 V.
Later, as disclosed in co-pending Canadian Patent Application Serial No. 2,163,187, filed Nov. 17, '1995, by a common applicant, it was discovered that similar polymerizable monomer additives could also be used as gassing agents in batteries for purposes of activating internal electrical disconnect devices on overcharge. Again, a preferred additive was biphenyl which was shown to generate a satisfactory amount of disconnect activating gas during overcharge of certain lithium ion battery embodiments.
Later still, as disclosed in co-pending Canadian Patent Application Serial No. 2,205,683, filed May 16, '1997, by a common applicant, it was discovered that certain additives could be used to make overcharged batteries safe by `automatically` discharging them to a safe state of charge. This was accomplished by choosing an additive which polymerized to form a conductive polymer during overcharge. As in the above Canadian Patent Application Serial No. 2,156,800, such an additive can initially form an ionically blocking film when it polymerizes and thus increase the internal resistance of the battery. However, when sufficient polymer has been generated to bridge the gap between cathode and anode electrodes, the electrically conductive polymer can then create a mild internal short in the battery thereby effecting a slow, safe self-discharge. This invention was particularly useful for batteries comprising activated internal electrical disconnect devices which can no longer be externally discharged to drain them of energy and put them into a more thermally stable state of charge. Again, a preferred additive for this purpose was biphenyl.
In the art, it is common to protect against certain abuse situations via use of a suitable separator that melts or shuts down at a specific temperature (the shutdown temperature). For instance, in European Patent Application No. 746050, Sony employs certain polymerizable electrolyte additives that apparently generate sufficient heat when polymerized during overcharge abuse that the separator melts and shuts down well before an unsafe state of charge is reached. The melting of the separator increases the internal resistance of the battery markedly and protects it against further overcharge. Microporous polyolefin separators are generally considered suitable for this purpose. In particular, microporous polypropylene and polyethylene separators, having shutdown temperatures of about 155.degree. C. and 125.degree. C. respectively, are commonly employed in lithium batteries. The embodiments described in the actual examples of the Sony disclosure employed polyethylene separators. The enabling electrolyte additives disclosed were limited to a variety of aromatic compounds all of which had structures consisting of single benzene rings with alkyl, alkoxy, and/or halogen groups substituted for hydrogen.
It has also been suggested in the art that certain redox or chemical shuttle additives can be employed in non-aqueous rechargeable lithium batteries for purposes of protecting against overcharge abuse. This mechanism is intended to be similar to the oxygen recombination reaction which can harmlessly consume overcharge current in aqueous batteries. In Japanese published patent application 07-302614, Sony proposes the use of various redox shuttle additives for lithium batteries. Thus, for the most part in this invention, the additives have to be capable of undergoing reversible oxidation /reduction cycles and so the oxidation and the reduction species of the additive must both be chemically stable. The enabling additives disclosed were limited to a variety of aromatic compounds having either a single ring, a multi-ring with common or shared bond, or a dimethoxybiphenyl base structure.
In the aforementioned prior art items pertaining to biphenyl additives, it was shown that use of a small amount of biphenyl did not seriously affect battery performance under normal single cell test conditions. However, the individual cells in battery packs comprising more than one cell in series often are subjected to voltages that slightly exceed the usual maximum operating charging voltage of a single cell (eg. by 0.1 V). At slightly higher voltages and/or temperatures, the additives can be expected to polymerize somewhat more and thus can be expected to affect battery performance more. Thus, while the aforementioned biphenyl and other additives may provide a satisfactory level of performance for some applications, additives which are somewhat more resistant to polymerization while still providing overcharge protection may be preferred in other applications where higher temperatures or voltages might be expected.